Very good question but it has a few issues that need to be addressed.
Breast asymmetry is THE NORM - not an exception. While we try to correct breast asymmetry as much as possible (using different fill in saline breast implants), we rarely succeed in getting perfectly symmetrical breasts. I suspect that the odds of perfect symmetry would be lower in your case where the chest wall is deformed. To get a chance at symmetry, you may need placement of a computer generated customized implant which would essentially correct the Pectus Excavatum and create a symmetrical plateau for the implants to sit on.
As regards your choice of implant - I seriously disagree. While you focused on the old studies purporting a much lesser rate of capsular contraction rate with textured implants you overlooked the facts that - Textured implants ripple more than smooth implants and leak much faster than smooth implants. Moreover, saline implants are not only heavier but also ripple more than silicone filled breast implants. If you wanted an implant that would demonstrate its ripples more than any other and leak sooner than all of them - the textured saline implant is your implant. A much better choice would be a smooth silicone gel implant ESPECIALLY with a thin breast coverage situation.
Finally, while a trans axillary approach is feasible, nothing beats the accuracy of a periareolar access and the re-usability, over and over, of its largely inconspicuous scar.